Equity (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended							0 Months Ended										9 Months Ended	0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Common Units	Sep. 30, 2012 Common Units	Dec. 31, 2012 Common Units	Sep. 30, 2013 Class B Units	Dec. 31, 2012 Class B Units	Jul. 02, 2014 Class B Units M & R item	Jul. 02, 2013 Class B Units M & R	Sep. 30, 2013 Maximum	Jun. 30, 2013 Maximum	Mar. 31, 2013 Maximum	Dec. 31, 2012 Maximum	Sep. 30, 2013 Minimum	Jun. 30, 2013 Minimum	Mar. 31, 2013 Minimum	Dec. 31, 2012 Minimum	Sep. 30, 2013 November 2012 ATM Common Units	Aug. 07, 2013 August 2013 ATM Common Units	Sep. 30, 2013 August 2013 ATM Common Units	Sep. 05, 2013 September 2013 ATM Common Units	Sep. 30, 2013 September 2013 ATM Common Units
Equity Offerings
Aggregate common units sold (in units)					0		16,112	22,408														9,300		5,900		900
Proceeds from public equity offerings, net					$ 1,039,849,000	$ 1,191,066,000																$ 584,300,000		$ 396,000,000		$ 59,500,000
Manager fees																						9,400,000		4,000,000		600,000
Common units							147,763		127,494	15,964	19,954		4,000
Conversion ratio													1
Number of installments												4
Number of anniversaries												3
Offering under program																							$ 400,000,000		$ 1,000,000,000
Common Unit Price (in dollars per share)														$ 72.35	$ 71.20	$ 61.97	$ 55.95	$ 65.27	$ 56.90	$ 51.77	$ 46.03
Distributions of Available Cash
Distribution per common unit (in dollars per unit)	$ 0.85	$ 0.84	$ 0.83	$ 0.82